Vanadium oxides on inorganic support materials constitute the most important supported metal oxide catalysts in both heterogeneous and homogeneous industrial applications [Weckhuysen B M, Van Der Voort P, Catana G (eds) (2000) Spectroscopy of transition metal ions on surfaces. Leuven University Press, Leuven]. In these catalysts, the amount of active vanadium species exposed to the reactants varies with the type of support and the loading of vanadium oxide on the carrier [Gao X, Wachs I E (2002) Top Catal 18:243]. Here both the surface area and the oxide type (i.e., textural and surface properties) dictate the maximum amount that can be loaded before surpassing the monolayer coverage leading to crystalline V2O5 formation [Weckhuysen B M, Keller D E (2003) Catal Today 78:25]. Typical examples of support materials are SiO2, Al2O3, ZrO2, and TiO2. The preferred choice of support material usually depends on the reactive environment of the catalyst during operation as well as the particular reaction in focus.
In the gas-phase oxidation of aqueous ethanol a V2O5 catalyst supported on TiO2 and immobilized on clay described in U.S. Pat. No. 5,840,971 was found to provide a very high selectivity of 97% towards acetic acid even at relatively moderate temperatures and pressures (180° C. and 1.7 bar) with the only byproduct being CO2. This was an interesting finding, especially due to the growing interest worldwide in production and utilization of bioethanol. Bioethanol has found main usage as fuel or fuel additive in the transport sector, though at present it is impossible to produce enough bioethanol to replace gasoline. Moreover, it is required that all water, which typically accounts for up to 95 wt %, is removed from crude bioethanol prior to its use in combustion engines to reach satisfying fuel utilization. Initial flash distillation will generally provide a fraction with about 50 wt % water remaining, while further water removal is a very energy demanding and costly process.
Alternatively it has therefore been suggested that ethanol is further converted into higher value-added chemicals via reactions that are not as sensitive to the water content [Rass-Hansen J et al., J. Chem. Technol. Biotechnol. 82 (2007) 329]. In this context an interesting possibility is to oxidize the ethanol whereby commodity acetyl derivatives such as, e.g. acetaldehyde, acetic acid and ethyl acetate can be formed. Of these possibilities only the formation of acetic acid has been demonstrated under commercially interesting conditions, and there is consequently a need for a relatively cheap and robust catalyst that may effect the conversion of aqueous ethanol such as bioethanol from fermentation to other industrially applicable derivatives such as acetaldehyde.
V2O5 catalysts supported on TiO2 are not only interesting in relation to oxidation processes. It is also well known that TiO2, especially in the anatase form, is an excellent support for vanadium oxides making highly active materials for the selective catalytic reduction (SCR) of nitrogen oxides (i.e. deNOx) by injection of ammonia in power plant flue-gases and other industrial off-gases [Parvulescu VI, Grange P, Delmon B (1998) Catal Today 46:233]. However, the activity of the industrial VOx/TiO2-based SCR catalyst is limited by the surface area of the anatase carrier, since only up to one monolayer of the vanadium oxide species can be accepted corresponding to a vanadia loading of 3-5 wt.%. Increased loading results in decreased deNOx activity and increased ability to oxidize NH3 and possibly also SO2 in the flue gas [Busca Get al. (1998) Appl Catal 18:1].
Kang M et al. in “Methyl orange removal in a liquid photo-system with nanometer sized V/TiO2 particle”, Journal of Industrial And Engineering Chemistry, vol. 11, no. 2, pages 240-247 discloses a nanometer-sized V/TiO2 photocatalyst synthesized by a commercial sol-gel method for the removal of methyl orange. However, the catalyst composition is mixed anatase/rutile which has inferior catalytic properties. Furthermore the BET surface area is low, ≤15 m2/g. The crystal size of the material is large (20-70 nm) and increases with increasing content of vanadium pentoxide. Crystalline vanadium pentoxide is observed in the material of Kang at a concentration of 10% w/w, possibly even at lower concentrations. Kang's material is not obtained using the seed/template technique, and the application in SCR deNOx reactions is not mentioned, nor is the SCR activity measured.
Bellifa A et al. in “Preparation And Characterization Of 20 wt. % V2O5—TiO2 Catalyst Oxidation Of Cyclohexane”, Applied Catalysis A: General, Elsevier Science, Amsterdam Vol. 305, No. 1, (2006) p. 1-6 discloses the preparation of a V2O5—TiO2 catalyst for the oxidation of cyclohexane. The catalyst is prepared by an acid-catalyzed sol-gel process (not using the seed/template technique), and exhibited a high oxidation activity and selectivity for the conversion of cyclohexane into cyclohexanol. The images obtained by TEM (transition electron microscopy) show nanoparticles in the range from around 20-300 nm.
However, just like Kang's catalyst, Bellifa's catalyst composition is mixed anatase/rutile which has inferior catalytic properties and a low BET surface area, 27 m2/g. It further contains large particles with a small fraction of nanocrystals app 5 nm and displays a very broad particle size distribution. Finally, the material of Bellifa et al. is calcined at a relatively low temperature (300° C.) which is expected to lead to structural changes of the catalyst particles caused by sintering when used for deNOx purposes, i.e. typically at 350-400° C. Bellifa's catalyst is therefore not suited for application in SCR deNOx reactions and the SCR activity is not measured.
Kumar V et al. in “An investigation of the thermal stability and performance of wet-incipient WO3/V2O5/TiO2 catalysts and a comparison with flame aerosol catalysts of similar composition for the gas-phase oxidation of methanol” Applied Catalysis B: Environmental, Elsevier, Vol. 69, No. 1-2, 2006 Pages 101-114, discloses WO3/V2O5/TiO2 catalysts which are prepared by incipient wetness impregnation and flame aerosol method. The catalyst prepared by flame aerosol method exhibits particles sizes between 28-45 nm and showed good catalytic performances in the oxidation of ethanol. However, like Bellifa's catalyst Kumars material displays a rather low BET surface area, 90 m2/g, and consists of crystals having different sizes up to 30 nm which renders the performance unpredictable, and most importantly, the high-surface area anatase particles of Kumar's material is not stabilized, which leads to a drastic decrease (about 70%) of the surface area when the material is impregnated, even at low V2O5 concentrations. Again no measurements of NOx SCR activity are reported.
A further interesting application of V2O5 catalysts supported on TiO2 is in the areas of catalytic combustion of volatile organic compounds (VOCs)
[Everaert K et al., Journal of Hazardous Materials B109 (2004) 113-139] and especially in the photocatalytic degradation of VOCs [Tanizaki T et al., Journal of Health Science, 53(5) 514-519 (2007)].
In the atmosphere, especially in indoor air, there are many kinds of volatile organic compounds (VOCs) which lead to water and air pollution, and even to indoor air pollution. Though their concentrations are generally at low ppb levels, some of them have a harmful influence on the living environment: for example, the odorous substances which are generated by the biological activity of human beings and other animals; the “sick building” chemicals vaporized from paint, and bonds used as building materials.
A promising approach for remediating VOC is to employ photocatalytic reactors that oxidize these compounds. Semiconducting materials contain electrons that are confined to relatively narrow energy bands. The band of highest energy that contains electrons is the valence band, while the band lying above the valence band, i.e. the conduction band, has very few electrons. The difference in energies between the highest energy of the valence band and the lowest energy of the conduction band is termed the band gap energy.
When a semiconductor absorbs a photon of energy equal to or greater than its band gap, an electron may be promoted from the valence band to the conduction band leaving behind an electron vacancy or “hole” in the valence band. If charge separation is maintained, the electron and the hole may migrate to the catalyst surface where they participate in redox reaction with sorbed species (Burns R. A. et al. (1999) Journal of Environmental Engineering, January, 77-85).
Photocatalytic oxidation of VOCs is a cost-effective technology for removal of VOCs compared with adsorption, biofiltration, or thermal catalysis. The most commonly used catalyst in this application is titanium dioxide (titania) because of its stability under most reaction conditions, and because TiO2 is an inexpensive, non-toxic and biocompatible material. Numerous studies have indicated that illuminating the anatase form of TiO2 with near UV radiation has been successful in eliminating organic compounds such as methanol, acetone, methyl ethyl ketone, isopropanol, chloroform and trichloroethylene (Alberici R. M. et al. (1997) Appl Catal B: Environmental, vol 14, 55-68; Dibble L. A. et al. Environmental Science and Technologies, vol 26, 492-495).
For the possible destruction of VOCs at room temperature, consideration has also been given to the photocatalytic oxidation of gaseous phase organic compounds, using TiO2-based catalysts coupled with other catalytically active materials. For example, has the photocatalytic oxidation of acetone with a pure TiO2 catalyst or a mixed TiO2/ZrO2 catalyst at 77° C., prepared in thin layer form using a sol-gel method, been described (Zorn M E et al (1999) Appl Catal B: Environmental, vol. 23, p. 1-8). Photocatalysts of Pt/TiO2 types have been used to decompose ethanol at a temperature in the region of 200° C. (Kennedy J C. et al. Journal of Catalysis, vol. 179, p. 375-389 (1998). A mixed photocatalyst of CdS/TiO2 type was tested for the decomposition of phenol, 2-chlorophenol and pentachlorophenol in the liquid phase (Serpone N et al. (1995) Journal of Photochemistry and Photobiology A: Chemistry, vol. 85, p. 247-255).
There is consequently a need for a relatively cheap and robust method of preparation, whereby high loadings of different catalytically active species on metal oxide carriers can be achieved without sacrificing catalytic selectivity.